Loudness constancy stems either from residual occluded-ear sound or cognitive knowledge of occlusion.

Evidence

In each of the studies in which binaural summation has been largely abolished, the second ear has been occluded by some means. The lack of summation could have two logical causes. First, there may be some residual sound energy at the occluded ear that is sufficient to maintain its contribution to binaural loudness. Second, there may be some cognitive element which generates loudness constancy through the knowledge that one ear is occluded. — Dijk, Pim van, Physiology, Psychoacoustics and…, 1525